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Resistor Tutorial

In the following post we will be co vering many aspects of resistors including: how they work, the color codes, the calculations, and the difference between putting them in series or parallel.

What is a Resistor?

A resistor is one of the most common and critical parts of any circuit. Its job is to restrict the flow of electrons through a circuit to keep your components from frying. There are many different types of resistors on the market such as plated through-hole (PTH), surface mount (SMD/SMT), potentiometers, and many others that are beyond the scope of this post. Here we will cover the two most common variants of the resistor, through-hole and potentiometers. SMD’s will be covered in a separate series of posts dedicated to surface mount devices.

Lets say you have a circuit where a 5 volt power supply is powering a simple LED. If the LED’s forward voltage is 3 volts and its maximum forward current is 50 mA then you will run into a problem. If the power supply is connected directly to the LED it will blow it and possibly damage the power supply. This is where a resistor comes into play, it will limit the current flowing through to the LED so that it can only draw as many Amps (or Milliamps) as the resistor will allow. So how do we select a resistor that only allows the LED to draw 50 mA at 3 volts? There is a fairly straight forward equation for this.

R = Vs – Vf / If Where R is resistance, Vs is the source voltage, Vf is the forward voltage, and If is the forward current.

Substituting known variables into the equation we get R = 5 – 3 / .050

R = 40 (we would use a 47 ohm resistor as it is the closest common resistor)

So we add a 47 ohm resistor into the circuit and the LED will be properly powered.

How do they work?

Not all resistors are created equally, but they all function the same way. Modern through-hole resistors are most commonly made by wrapping a carbon, metal, or metal oxide film around a plastic, ceramic, or fiberglass core. This creates a semi-conductive environment that can be precisely made to limit electrons to almost exact specifications. While all resistors are manufactured in X ohm increments they come in a 5% or 10% tolerance rating, meaning that the exact Ohms of any given resistor will be within 5% or 10% of the Ohms they are advertised at. To find out what the tolerance rating is we can look at the colored bands on the outside of our resistors.

The color code:

Every through-hole resistor is manufactured with colored bands around it, most commonly either four or five of them. The easiest band to remember is the last one, it will almost always either be silver or gold. Silver tells us that the resistor is manufactured with a 10% tolerance rating, gold represents 5%. The rest of the bands get a bit more complicated. If you don’t remember the color code the easiest way to find out what a certain combination is would be to google a resistor color code calculator. Below is a chart covering the meanings of each colorfor both 4 and 5 band resistors.

credit for this chart goes to www.imajeenyus.com

Series vs. Parallel

How you use a resistor is almost as important as having a resistor in the first place. There are two main ways you can wire a circuit, either in series or parallel. This concept can be overwhelming at first but when broken down it is relatively easy to understand. Lets first take a look at how a circuit works when wired up in series.

Series

figure 1

At 9 volts, a 1.8 ohm resistor will draw 5 Amps. The current runs from the point of highest voltage (positive side of the battery) to the point of lowest voltage (negative side of the battery) passing through the resistor on its way. If we were to put a second 1.8 ohm resistor in line with the first we would effectively double the amount of resistance in the circuit.

figure 2

Note: Each colored wire in the diagram to the left is referred to as a “node”

As you can see in the above figure, when connecting multiple resistors of the same value together (in series) the amount of resistance is doubled, which in turn cuts the amperes in half. Things Don’t change much when using resistors of different values, the equation to solve for your total resistance is quite simple.

X = R + R + R…. where X is the total resistance and R is each resistor.

So to sum up using resistors in series, as long as each resistor is connected together then the total resistance is equal to the sum of all the resistors.

Parallel

Wiring a circuit in parallel is quite a bit different that doing so in series, but can be much more powerful if used effectively. To understand how these types of circuits work you must first understand how circuits in general work. The flow of electricity follows all available paths to reach the point of lowest voltage. So if you wire up multiple different ways to get to the negative terminal of a battery, electricity would flow through all of them from the positive terminal. Lets take a look at an example of this.

As you can see above the current is flowing through both resistors, but in a different manner than before. In this circuit the resistors are each pulling 5 Amps, which means there is a total of 10 Amps in the circuit. By doubling the 1.8 Ohm resistor we’ve actually cut the resistance in half, which in turn doubles the amperage through the circuit. This is important to remember and can be easily be confused, in parallel the resistance is cut in half and the amperage is doubled, while in series the resistance is doubled, so the amperage is cut in half.

Things change here when you start using resistors of different values in parallel, but the math to find your total resistance is always the same

X = R1*R2 / R1+R2 Where X is the total resistance and R1 R2 are the two resistors. If we have more than two resistors in parallel then you can use this method to find the value for X, then substitute that for R1. The new equation would be Y = Rx*R3 / Rx+R3.

If you have a lot of resistors in parallel the above equation can get tedious so we can use reciprocal method to find the total resistance:

1/R = 1/R1 + 1/R2 + 1/R3 + 1/R4

By dividing each of the resistors values by 1 then dividing that total by 1 we get the total resistance.

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